ATLATA (CBS Atlanta) – Medical marijuana use has cut overdoses on pain killers by 25 percent, says a new study.

Researchers looked at death rates caused by overdoses of opioid painkillers, like OxyContin or Vicodin, between 1999 and 2010 using data from the Centers for Disease Control and Prevention.

They found that overdose deaths increased in all 50 states during that period of time. But, in the 13 states that have laws allowing the use of marijuana for medical reasons, the death rates from overdoses were about 25 percent lower when compared to the rest of the states.

In 2010 there would have been about 1,700 fewer deaths than would have been expected if those states did not allow medical marijuana.

It’s not clear why there would be fewer opioid overdoes deaths, but the numbers offer evidence for the benefits of using marijuana to treat pain.

It is possible that patients may use medical marijuana, instead of prescription painkillers, to treat their chronic pain, the researchers said.

There also seemed to be a benefit over time. During the study period, states with medical marijuana laws saw overdose deaths reduced by 20 percent in the first year after they were enacted. After five years the rate dropped by 34 percent.

“You can often find very suggestive correlations that turn out to be not true, and are controlled by other variables,” Hayes told Live Science. “But in this case, by looking at the number of years following implementation [of the law], that means that rather than just having a snapshot of the correlation at some time, they were able to actually find a continuation of the trend over years.”

Death rates from drug overdoses have more than tripled since 1990, according to the CDC. Nearly three quarters of prescription-drug overdoses are caused by opioid painkillers.